The present invention relates to films made of nonoriented polyester, and more specifically to nonoriented coextruded films which further include a heat seal layer which comprises both copolyester and a polyolefin.
Biaxially oriented polyethylene terephthalate, commonly referred to as "oriented polyester" or "OPET", is a common packaging film with over 200 million lbs. used annually. OPET is used for a wide variety of packaging applications, both alone and in combination with other materials (lamination) such as snack food packaging, adhesive labels, lidding, pouches, medical device packaging, and packaging for luncheon meat, cheese, bacon, and the like.
OPET films are manufactured by an orientation process utilizing tenter frame or "double bubble" techniques. Tentering of plastic films is illustrated in U.S. Pat. No. 2,823,421 and double-bubble orientation is illustrated in U.S. Pat. No. 3,555,604. U.S. Pat. Nos. 4,141,736 and 4,207,363 describe oriented films made using the tenter and double bubble processes, respectively. The orientation process comprises the steps of (1) extruding a thick web and cooling to a solid state; (2) precise reheating of film to the "orientation" temperature, (3) stretching the film in both longitudinal and transverse directions, and (4) reheating the film to relieve internal stresses. The process of orientating polyester is very complicated, requiring very expensive equipment and several highly skilled operators.
This is in contrast to the process used to make many other commodity plastic packaging films such as polyethylene. Polyethylene film is commonly produced by blown or cast film extrusion techniques. Such films, while being easier to make, do not have the properties of toughness or stiffness which make polyester a favored packaging film for use in flexible packaging applications.
Prior to this invention and recent developments of new polyester resins, polyester film produced without orientation yielded a very brittle film of limited use in flexible packaging. Various modifications have been made to nonoriented polyester films to improve its properties. One example where the polyester is modified to improve its processability in slow cooling of thick films is disclosed in U.S. Pat. No. 4,175,147. This patent discloses single layer nonoriented films comprised of 97.5% to 99.9% w/w of PET blended with 2.5%-0.1% of polycarbonate. Such films, however, are not described as having any toughness above that of nonoriented 100% PET films of similar thicknesses and intrinsic viscosities.
Oriented polyester films often include a heat sealant layer, and are then used in many flexible packaging applications. Oriented heat sealable polyester films are commercially available whereby the heat sealant is applied as a coating. OL MYLAR and SCOTCHPAK are examples of heat sealable OPET films having the heat sealant applied as a coating. OL MYLAR is a registered trademark of E. I. DuPont de Neumours and Company. SCOTCHPAK is a registered trademark of The 3M Company.
Most heat seal coatings consist of a polymer dissolved in a solvent such as methyl ethyl ketone, toluene or ethyl acetate. The solutions are applied as a dispersion. The solvent is subsequently evaporated by passing the web through an oven. This necessitates the need for disposing of or recovering the often hazardous solvents. Furthermore, often a small amount of the solvent remains with the coating, commonly referred to as "retained solvents". Retained solvents are a concern when packaging food or medical products.
MELINEX 850 is an example of another heat sealable OPET film. MELINEX is a registered trademark of Imperial Chemical Industries, Ltd. It is belieVed that U.S. Pat. No. 4,375,494 discloses the method of making MELINEX B50. The film is made by coextruding a polyester base layer and a heat seal layer, orienting the coextruded layers using biaxially orientation techniques, and then heating the resulting film to a temperature below the crystalline melting point of the oriented polyester but at which the heat seal layer looses its orientation to become an amorphous heat sealable layer.
Heat seal layers are also known on nonoriented multilayer films, including PET, such as that disclosed in U.S. Pat. No. 4,389,450. This patent discloses a multiple layer heat sealable packaging sheet. The base layer (nonsealant layer) comprises a blend of 25%-75% of a polymer with coefficient of friction (COF) of less than about 2, including ethylene copolymers, polyester, polypropylene nylon. The blend also includes 25%-75% of a polymer with a COF greater than 2, including ethylene acrylic acid, ethylene methylacrylate, ethylene ethyl acrylate, ethylene vinyl acetate, and ionomer. Films made according to U.S. Pat. No. 4,389,450 containing 25% olefinic material blended into polyester do not yield the toughness of films of the present invention.
In application Ser. No. 06/759,391, the grandparent application for this case, now U.S. Pat. No. 4,765,999, a nonoriented coextruded heat sealable film is disclosed. A copolyester used in the heat seal layer was found to provide exceptional heat seal properties, especially when the film was laminated to a paper web and used to make microwave popcorn popping bags. For that use, the base polyester layer was not required to have, nor did it haVe, any particular toughness characteristics when made into thin films. In copending application Ser. No. 07/050,166, one of the parent applications for this case, nonoriented tough polyester films are disclosed. The coextrusion of such films with heat seal layers (as discussed below) is also disclosed. In copending application Ser. No. 07/067,672, the other parent application for this case, the use of new resins for films similar to those described in Ser. No. 07/050,166 are disclosed.
Heat sealable films are often laminated to other materials for particular applications. For example, two common means of packaging are the use of a heat sealed pouch (such as in powdered soft drink mixes) and plastic containers with a peelable lid (such as in yogurt, individual servings of jams and jellies, etc.) The pouch or lidding material is often a composite structure formed by (1) laminating printed paper or aluminum foil to oriented PET film and (2) applying a heat seal coating to the laminated structure.
The lamination is typically made by placing an adhesive layer on the paper, aluminum foil, or even the oriented PET film and then laminating the two webs of material together. Such adhesive lamination techniques are well known. An alternative to adhesive lamination is extrusion lamination whereby the webs are bonded by a polymer extruded hot onto one of the webs. Extrusion lamination techniques are also well known.
The second step in producing typical flexible packaging stocks is application of the heat sealant by means of coating. The sealant coating can be applied by dissolving a heat sealable polymer in a solvent and applying it to the surface of the laminate. The laminate is then passed through an oven to evaporate the solvent. The polymer, which does not evaporate, forms a uniform heat seal coating on the structure. A small amount of retained solvent often remains in the coating just as when a heat seal coating is applied to single layer OPET.
Besides the problem of retained solvents, such laminates are also expensive in that they use oriented films and require two converting steps to produce.
A leading candidate for a dual ovenable (convection and microwave) container for storing and cooking food products is crystallized copolyester (CPET). Trays of this material are manufactured by Mullinix Packages Incorporated, 3511 Engle Road, Fort Wayne, Ind. 46809. A lidding stock for such a tray must be heat sealable, but the heat seal strength must not be greater than internal layer strengths, such as lamination layer strengths, or the lidding stock will delaminate rather than peel off the tray.